Endoluminal prostheses, also known as stent-graft prostheses, are medical devices commonly known to be used in the treatment of diseased blood vessels. A stent-graft prosthesis is typically used to repair, replace, or otherwise correct a damaged blood vessel. An artery or vein may be disease or damaged in a variety of different ways. The stent-graft prosthesis may therefore be used to prevent or treat a wide variety of defects such as stenosis of the vessel, thrombosis, occlusion, or an aneurysm.
Stents are generally tubular open-ended structures providing support for damaged, collapsing, or occluded blood vessels. They are radially expandable from a radially compressed configuration for delivery to the affected vessel site to a radially expanded configuration when deployed at the affected vessel treatment site, with the radially expanded configuration having a larger diameter than the radially compressed configuration. Stents are flexible, which allows them to be inserted through, and conform to, tortuous pathways in the blood vessels. Stents are generally inserted in the radially compressed configuration and expanded to the radially expanded configuration either through a self-expanding mechanism, or through the use of a balloon catheter. Helical stents are formed of a continuous, helically wound wire typically having a series of struts and apices, also known as crowns or bends. In some helical stents, connectors are disposed between adjacent bands of the helically formed wire.
A graft is another type of the endoluminal prosthesis, which is used to repair and replace various body vessels. Whereas the stent provides structural support to hold a damaged vessel open, a graft provides an artificial lumen through which blood may flow. Grafts are tubular devices that may be formed of a variety of materials, including textile, and non-textile materials. One type of non-textile material particularly suitable for use as an implantable prosthesis is polytetrafluoroethylene (PTFE). PTFE exhibits superior biocompatibility and has a low thrombogenicity, which makes it particularly useful as vascular graft material in the repair or replacement of blood vessels. In vascular applications, the grafts are manufactured from expanded PTFE tubes. These tubes have a microporous structure that allows natural tissue ingrowth and cell endothelization once implanted in the vascular system. This contributes to long-term healing and patency of the graft.
Stents and graft may be combined to form a stent-graft prosthesis, providing both structural support and an artificial lumen through which blood may flow. A stent-graft prosthesis is particularly useful to isolate aneurysms or other blood vessel abnormalities from normal blood pressure, reducing pressure on the weakened vessel wall, thereby reducing the chance of vessel rupture while maintaining blood flow. A stent-graft prosthesis is placed within the aneurysmal blood vessel to create a new flow path and an artificial flow conduit through the aneurysm, thereby reducing the exertion of blood pressure on the aneurysm. The stent-graft prosthesis incorporates one or more radially expandable stent(s) to be radially expanded in situ to anchor the tubular graft to the wall of the blood vessel at sites upstream and downstream of the aneurysm. Thus, endovascular stent-graft prostheses are typically held in place by mechanical engagement and friction by the outward radial force imparted on the wall of the blood vessel by the self-expanding or balloon expandable stent.
As previously described, polytetrafluoroethylene (PTFE) is a polymeric material that is well suited for use as the graft material of a stent-graft prosthesis. It is known in the art to form a stent-graft prosthesis that includes an outer PTFE layer which covers or lines at least one stent, as described in, for example, U.S. Pat. No. 5,700,285 and U.S. Pat. No. 5,735,892 to Myers. It is also known in the art to form a stent-graft prosthesis that includes an inner PTFE layer, an outer PTFE layer positioned about the inner PTFE layer and at least one stent interposed or encapsulated between the inner and outer PTFE layers, as described in, for example, U.S. Pat. No. 6,673,103 to Golds et al. and U.S. Patent Publication No. 2014/0130965 to Banks et al.
The design of a stent-graft prosthesis must balance strength and flexibility. Generally, increasing the strength a stent-graft prosthesis reduces its flexibility. Conversely, increasing the flexibility of stent-graft prosthesis generally decreases its strength. A stent-graft prosthesis formed with a helical stent without connectors and a PTFE graft material is generally highly flexible, but generally provides little support from either the helical stent or the fused PTFE layers when the stent-graft prosthesis is in tension. As the stent-graft prosthesis encounters tension during use or insertion within a delivery system, durability issues of the PTFE layers can occur. During the periods of tension, the stent-graft prosthesis elongates and the PTFE layers can tear. Pull-loading of a stent-graft prosthesis of this kind into a delivery system is therefore difficult and potentially damaging to the stent-graft prosthesis. Thus, the surgical team will often resort to push-loading of the stent-graft prosthesis into the delivery system. However, push-loading of the stent-graft prosthesis can result in train-wrecking issues causing damage to the stent-graft prosthesis.
Accordingly, there is a need for an improved stent-graft prosthesis design providing additional strength when the stent-graft prosthesis is under tension while maintaining flexibility, and methods for manufacturing such a stent-graft prosthesis.